1. Field of the Invention
The invention relates to a radiation image capturing device that captures an image of radiation transmitted though a human body and a radiation image capturing method.
2. Description of the Related Art
In the medical treatment field, a portable radiation image capturing device such as a flat panel detector (FPD) that detects strength of radiation transmitted through a human body and captures an image of an inner part of the human body has been used. Since the FPD (hereinafter, referred to as an electronic cassette) can capture an image in a state in which a patient is laid on a bed and adjust an image capturing location by changing the position of the electronic cassette, image capturing can be executed with flexibility with respect to a patient who cannot move.
Meanwhile, in the electronic cassette, even in a state in which radiation is not irradiated, charge is generated by a dark current and the charge is accumulated in each pixel. Since the dark current may appear as noise of a radiation image, the electronic cassette generally performs an operation of removing the charge accumulated in each pixel of the electronic cassette, immediately before a radiation image is captured. When the radiation image is captured, a control device instructs the electronic cassette and a radiation device to irradiate radiation to capture the radiation image. When capturing of the radiation image is requested, the radiation device starts radiation irradiation, and the electronic cassette starts exposure. If the radiation irradiation ends, the electric cassette reads the charge that is accumulated by the radiation irradiation. At this time, irradiation timing of the radiation by the radiation device and exposure timing of the electronic cassette are synchronized with each other. That is, synchronization of image capturing timing is carried out.
Japanese Patent Application Laid-Open (JP-A) No. 2010-081960 discloses technology in which a first measurement unit to measure time is provided in a console (control device), a second measurement unit to measure time synchronized with the first measurement unit is provided in an electronic cassette, a radiation device irradiates radiation during a predetermined period in a case in which the time becomes an exposure start time preset by the console, and the electronic cassette reads the charge generated in a radiation detector, after the predetermined period passes from the exposure start time.
The control device and the radiation device need to be electrically connected to carry out the synchronization of the image capturing timing. In order to electrically connect the control device and the radiation device to each other, a service person of a manufacture needs to perform connection work at the time of installing a system, and a service cost and a maintenance cost are needed. In a case in which a manufacture of the control device and a manufacture of the radiation device are different from each other, electric connection may not be performed in a lot of cases, in terms of safety. In contrast, in a case in which the control device and the radiation device are not electrically connected, the synchronization of the image capturing timing is not carried out. In this case, the electronic cassette is made to perform exposure for a time longer than an irradiation time of radiation, the radiation is irradiated while the electronic cassette performs the exposure, and the electronic cassette completely exposes the irradiated radiation and can capture a radiation image.
As such, in a case in which the control device and the radiation device are not electrically connected, the synchronization of the image capturing timing is not carried out. In this case, an operation of removing the unnecessary charge accumulated in each pixel of the electronic cassette may not be executed at an appropriate timing such as immediately before image capturing. At this time, a radiation image that includes a lot of noise occurring due to the dark current may be obtained.
Therefore, in order to remove a disadvantage due to the noise, the operation of removing the accumulated unnecessary charge may be repeated, regardless of the image capturing timing. Meanwhile, even in an electronic cassette in which image capturing is not scheduled in the near future (in a so-called standby state), the operation needs to be continuously executed. In particular, in a portable electronic cassette where battery capacity is restricted, if the operation is frequently executed, power is wasted, and the number of radiation images that can be obtained decreases. For this reason, convenience of the electronic cassette is lowered.
In recent years, a radiation detector such as a flat panel detector (FPD), in which a radiation-sensitive layer is disposed on a thin film transistor (TFT) active matrix substrate and radiation can be directly converted into digital data, has been put into practice. A radiation image capturing device that captures radiation images expressed by irradiated radiation using the radiation detector has also been put into practice. As methods of converting radiation in the radiation detector used in the radiation image capturing device, an indirect-conversion method that converts radiation into light in a scintillator and converts the light into charge in a semiconductor layer made of a photodiode and a direct-conversion method that converts radiation into charge in a semiconductor layer made of amorphous selenium are known. In these respective methods, there are various materials that can be used in the semiconductor layer.
Meanwhile, in the radiation detector, since a dark current causing noise exists, an image capturing time cannot be increased at random. For this reason, the radiation image capturing device that uses the radiation detector according to the related art exchanges various signals with a radiation generating device to irradiate radiation, and an irradiation operation of irradiating the radiation from the radiation generating device and an operation of capturing an image by the radiation image capturing device are synchronized with each other.
However, in a case in which the radiation irradiation operation by the radiation generating device and the image capturing operation by the radiation image capturing device need to be synchronized with each other, the radiation generating device and the radiation image capturing device need to be constructed as one integrated system. In a case in which the system is constructed by a combination with an existing radiation generating device, modification to carry out the synchronization needs to be executed at the side of the radiation generating device.
As technology that can be applied to enable capturing of a radiation image without synchronizing the radiation irradiation operation by the radiation generating device and the image capturing operation by the radiation image capturing device, Japanese Patent Application Laid-Open No. 2010-264181 describes technology for detecting a start of irradiation of radiation, on the basis of a bias current.
However, according to the technology that is described in Japanese Patent Application Laid-Open No. 2010-264181, detection of the radiation irradiation needs to be performed at all times to detect the start of irradiation of radiation and perform image capturing. In a case in which the radiation irradiation is detected at all times, power consumption increases. In particular, in a portable radiation image capturing device (hereinafter, referred to as an “electronic cassette”) in which a radiation detector is incorporated and capturing of a radiation image is performed with power driven by a battery, if the power consumption increases, driving time may be shortened.
Therefore, in a case in which a radiographer instructs the radiation image capturing device to transit to an image capturing mode in which a start of irradiation of radiation is detected and a radiation image is captured, this causes more trouble for the radiographer.